The present invention pertains to the radio communication art and, more particularly, to a means for synchronizing transmissions from a plurality of sites.
The present invention finds particular application in a mobile two-way communication system, such as is shown in FIG. 1. Here, a base station 10, which includes a transmitter 12 and a receiver 14 transmits and receives over an antenna 16. The base station 10 transmits to a plurality of mobiles, two of which are shown at 20, 30. Each mobile, such as mobile 20, is comprised of a transmitter 22, a receiver 24, an antenna 26 and a transmit-receiver antenna switch 28.
In a particular two-way system, transmissions from the base station 10 to the mobiles are carried over a first channel, channel A, whereas transmissions from the mobiles to the base stations are carried on a second channel, channel B. The mobiles 20, 30 share channel B on a contention basis. This means that a mobile requesting transmission on a given channel will be allowed to transmit immediately if the channel is free, but will have to wait until the channel is free if the channel is already in use.
In many applications, the information signals communicated between the base station and the mobiles are encoded. Thus, the encoded information may be scrambled speech, digital data, and so forth. In the preferred embodiment of the instant invention, the encoding process is comprised of differential phase shift keying of digital data. Encoding and decoding of the data is accomplished at the base by an encoder/decoder 18 and in the mobiles 20, 30 by encoder/decoders 23, 33, respectively.
Encoding schemes for digital data generally include using an encoding rate determined by a transmit clock time base. Thus, proper decoding of an encoded transmission requires recovery of the clock transmit time base.
FIG. 2 is a block diagram of a conventional clock recovery and transmit time base system as may be found in mobile stations, such as mobile 20, according to the prior art. Here, a transmission from the base 10 over channel A is picked up by antenna 26 and routed via antenna switch 28 to the receiver 24. For transmissions which have been differentially phase shifted keyed, the output from receiver 24 is passed through an asynchronous detector 40, of conventional design, which waveshapes the received data in preparation for data recovery.
The output from the asynchronous digital phase shift key detector 40 is applied to one input 50a of a clock recovery phase lock loop 50. Clock recovery phase lock loop 50 is comprised of a conventional phase comparator 52 and a variable frequency divider 54. Phase detector 52 includes first and second inputs of 52a, 52b for receiving two input signals and an output 52c for producing an error signal representative of the phase difference of the signals applied at its inputs 52a, 52b. The first input 52a of phase comparator 52 is connected to the first input 50a of the clock recovery phase lock loop 50. The second input 52b of phase comparator 52 is connected to the output 54a of the variable divider 54. Variable divider 54 responds to signals at its control input 54b to divide signals applied at its input 54c by one of three divisors, i.e. k, k+1 or k-1. The error output signal appearing at the phase comparator output 52c is fed back to the control input 54b of variable divider 54.
A master clock oscillator 60 is provided which produces an output signal of frequency k.times.fo, where k is a selected integer. The output from the master clock oscillator feeds to the input 54c of the divider 54 through the second input 50b of the clock recovery phase lock loop 50. The output 50c of the clock recovery phase lock loop is taken from the output 54a of the variable divider 54.
In operation, the phase comparator compares the divided master clock oscillator signal to the output from the asynchronous DPSK detector and produces an output error signal at its output 52c which, when coupled back to the control input 54b of the variable divider 54 tends to phase lock the master clock signal to the output from the asynchronous detector 40. Thus, the signal appearing at the clock recovery phase lock loop output 50c is at frequency fo and it is synchronized to the transmit time base clock. Hence, it may then be used in the conventional manner to fully decode the encoded transmission.
For transmission by the mobile 20, the antenna relay 28 couples the antenna 26 to the transmitter 22 (FIG. 1) and the master clock 60 through a second fixed divider 80, of fixed divisor k, provides the transmit clock frequency fo. The data at the mobile is then encoded and transmitted over channel B to the base station 10 where it is then decoded by encoder/decoder 18 (FIG. 1). As with the mobile station, proper decoding by the base requires recovering the time base clock of the transmission.
A substantial problem with two-way communication systems, such as that shown in the prior art, is that synchronization at the receiving site must be established before the information transmitted can be recovered. The synchronization period often requires a substantial length of time, thus resulting in reduced efficiency and throughput.